1. Field of the Invention
The present invention relates to processes for manufacturing dielectric films for use in semiconductor devices; and more particularly to nitrogen containing silicon dioxide (SiO.sub.2) dielectric films.
2. Description of Related Art
High quality dielectric films have been made in the prior art by thermal nitridation of thin SiO.sub.2 films, resulting in nitrogen containing SiO.sub.2, known as oxynitride and also known as nitrided SiO.sub.2, having improved dielectric integrity, relieved strain and stress associated with volume expansion of SiO.sub.2, and hot-carrier reliability due to the incorporation of nitrogen at the Si--SiO.sub.2 interface, as compared to conventional thermal SiO.sub.2. Rapid thermal oxidation of Si in N.sub.2 O has been studied as a good way to incorporate nitrogen in a gate SiO.sub.2 dielectric due to process simplicity and the absence of simultaneous incorporation of hydrogen. See, Green, et al., Appl. Phys. Lett., 65, 848 (1994), Tang, et al., Appl. Phys. Lett., 64, 3473 (1994), Hwang, et al., Appl. Phys. Lett., 57, 3 (1990) and Fukuda, et al., Electron Lett., 26, 1505 (1990). However, N.sub.2 O-based oxynitrides incorporate such low levels of nitrogen in the films, that the resulting films may not be able to prevent boron penetration or other problems. See, Joshi, et al., IEEE Electron Dev. Lett., 14, 560 (1993).
An earlier study of an N.sub.2 O furnace based oxynitridation process focusing on the vapor phase reactions, concluded that nitric oxide (NO) is the critical species responsible for nitridation in the N.sub.2 O oxynitride process. See, Tobin, et al., Dig. Tech. Papers, 1993 Symp. of VLSI Technol., 51 (1993). Nitridation of SiO.sub.2 with NO in a conventional furnace environment indeed has been found to provide a higher nitrogen content at the Si--SiO.sub.2 interface. See, Okada, et al., IEEE Trans. Electron Devices, 41, 1608 (1994) and Okada, Dig. Tech. Papers, 1994 Symp. of VLSI Technol., 105 (1994). Recent reports on the rapid thermal technique demonstrate the advantages obtained with NO nitridation while simultaneously maintaining a low thermal budget. See, Harrison, et al., MRS Symposia Proceedings, 342, 151 (1994), Yao, et al., Appl. Phys. Lett., 64, 3584 (1994), Bhar, et al., IEDM Tech. Dig., 329 (1994), Bhat J., et al., IEEE Electron Dev. Lett., 15 421 (1994).
Direct oxidation in a pure NO ambient is, however, highly self-limited due to the strong Si--N bond and high nitrogen concentration in the NO-grown films. For example, in one experiment NO oxidation at 1100.degree. C., the oxide thickness was only 3.8 nm after 180 seconds. To alleviate this limitation, several two-step processes in which an initial oxide was grown in O.sub.2, and then annealed in NO were proposed for oxide thicknesses exceeding 5 nm. See, Okada, et al., IEEE Trans. Electron Devices, 41, 1608 (1994) and Bhat, et al., IEEE Electron Dev. Lett., 15 421 (1994).